Process for improving planographic offset printing plates

ABSTRACT

A PROCESS IF IMAGE REPRODUCTION WHICH COMPRISES: (A) IMAGE-WISE EXPOSING A SILVER HALIDE EMULSION ON A SUPPORT; (B) FORMING A METALLIC SILVER SURFACE IMAGE IN THE UNEXPOSED AREAS BY SILVER TRANSFER DEVELOPMENT; (C) TREATING THE SURFACE OF THE ELEMENT WITH A SOLUTION OF AN ADJUVANT WHICH RENDERS THE UNEXPOSED AREAS MORE OLEOPHOBIC THAN THE EXPOSED AREAS; AND (D) USING THE ELEMENT TO PRINT BY LITHOGRAPHY.

United States Patent 1' fice 3,592,547 Patented July 13, 1971 3,592,647 PROCESS FOR IMPROVING PLANOGRAPHIC OFFSET PRINTING PLATES Ralph Kingsley Blake, Westfield, N.J., and Michael P. Dunkle, Towanda, Pa., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del. N Drawing. Filed June 7, 1967, Ser. No. 644,092

Int. Cl. G03f 7/02 US. CI. 96-33 8 Claims ABSTRACT OF THE DISCLOSURE The making of superior planographic printing elements from light-sensitive silver halide coated elements requires a delicate balance of a variety of surface characteristics of the elements, some of which tend to conflict with others. The surface must accept and retain a greasy image only in the desired areas when an aqueous fountain solution is used. It must permit the making of erasures, when necessary, without causing halos or other undesirable images to appear. The image must remain sharp throughout prolonged use, and not tone. Stop-go characteristics, that is, the ability to print satisfactory copies after a period of machine shutdown, must also be good. These and other characteristics must be properly balanced to provide a superior element, due regard being had for ease of manufacture and overall cost.

The improved lithographic printing plates of this invention are prepared by a process which comprises forming a metallic silver image in the least exposed or unexposed areas of the surface of a light-sensitive photographic element by silver transfer development, and treating said surface of the element with a liquid solution containing an adjuvant having a hydrophilic group to enhance the oleophobic character of the metallic surface image. When printing from these elements, the surface of the element is treated with an aqueous fountain solution, and an ink having an oleophilic binder is applied to the treated element whereby said greasy ink attaches to the background area (original exposed area) and is repelled by the metallic silver surface image formed by silver transfer development. The inked element is then used to print onto any suitable ink-receptive surface.

This invention includes any process in which a silver image is formed at the surface of the colloid layer by means of silver transfer development (sometimes called solution physical development) within said layer and treatment of said image with an adjuvant to make this silver surface image oleophobic, or more oleophobic, and ink repellent. It is not restricted to the processes disclosed in the following specific description and examples.

When a light-sensitive silver halide element, e.g., a gelatino-silver halide film, is exposed and developed with low-solvent conventional developers, the surface of the exposed areas of the element is made relatively oleophilic in comparison to the unexposed surface area. However, when a silver halide solvent and nucleating agent are added in the proper amounts to the above type of developers, silver transfer takes place in the unexposed areas and a silver image is formed at the surface of the layer providing the developer is sufiiciently rapid in its action. A two-agent developer of the super-additive type is preferred, particularly in a case of a film which is classified in the art as a slow-developing film. The developer may be aided by increasing the pH of the solution, which in itself increases the developing rate.

The element, after the silver transfer development described above, is contacted with an adjuvant to make the surface silver image more oleophobic than the surfaces of the exposed regions. It does not matter generally whether the silver surface image is oleophilic or oleophobic in relation to the background or exposed surface areas before the treatment with the adjuvant; the adjuvant will cause the silver surface image to be more oleophobic than the exposed areas in either instance.

The process of this invention involves increasing the oleophobic properties of a silver image, produced by silver transfer development, by treating the silver surface image with an adjuvant. The adjuvant is described as on having the necessary adsorbent and oleophobic groups as defined herein. The adjuvant is such that, when it is placed in contact with silver it becomes integrated with the silver in the silver surface image, and makes the un exposed areas oleophobic relative to the exposed areas due to its hydrophilic properties. By defining the adjuvant as one that becomes integrated with the silver surface image, it is intended to indicate that the adjuvant is adsorbed or bound to the silver of the silver surface image, but not necessarily reacted chemically with the total mass of the silver. An organic adjuvant which becomes integrated with the silver surface image should contain in its molecule one or more groups having an affinity for silver. These groups generally comprise the following:

(1) SH or a group enolizable to SH;

(2) SR where R is an easily hydrolyzable group to give SH, such as an acyl, amidinium or NHR where R is an alkyl, aryl, aralkyl, cycloalkyl, or heterocyclic group;

(3) a thioacid group;

(4) a thioamide group;

(5) a selenium analogue of the above four groups.

For the compound to produce the desired oleophobic properties in the silver, the adjuvant must also contain in addition to the above one or more hydrophilic groups, e.g., -COOH, OH, NH which are sufficiently hydrophilic to overcome the effect of any of the groups in the molecule which have oleophilic properties, e.g., fatty chains of several carbon atoms, aromatic rings, etc.

Adjuvants possessing these properties include the following:

(l) thiouracil (2) mercapto-succinic (3) thioacetic acid (4) 2-thiouracil-5-carboxylic acid (5) l-thioglycerol (6) o-mercaptobenzoic acid (7) 6-methyl-thiouracil (8) Z-mercaptomethyl amine hydrochloride, etc.

The oleophobic properties of a silver surface image, produced by silver transfer development, can also be improved by treating said silver surface image with an inorganic adjuvant, e.g., tantalum potassium fluoride, said inorganic adjuvant only becomes integrated with the silver in the silver surface image, thus making the unexposed areas oleophobic in relation to the exposed areas.

In this invention, the silver halide emulsion layer is preferably overcoated with a transparent receptive layer, sometimes called the antiabrasion layer, which is normally a gelatin layer. It is into this layer that the nucleating agents will permeate to allow the surface silver layer to form. The receptive layer should be a hardened nonremovable colloidal layer which is permeable to water. The receptive layer may be nucleated after exposure and during treatment in the developing solution. Some lightsensitive elements contain silica in this gelatin overcoating, known as the antiabrasion layer, but the silica is not es sential to the process. If the light-sensitive element does not contain an antiabrasion layer, the silver surface image will form on and in the top portion of the emulsion layer of the element during development with a nucleating agent.

In the unexposed areas during development, it is believed that the silver ions migrate to or near the surface of the light-sensitive element as complex ions. The nucleating agents, having permeated the surface region, are the starting points for reduction of the silver ion complex to metallic silver. If too much silver halide solvent action occurs and insufficient nucleating agent, e.g., sodium sulfide, is present, the metallic silver may diffuse to the surface and externally plate out as a silver image only on and not in the surface which is very easily abraded. When the solvent action is too low and the nucleating agent concentration too high, the sulfide ions apparently migrate too far into the emulsion and fog the individual silver halide grains. This fogging forms black silver deep within the emulsion, and the area at and near the surface above said silver cannot be made more oleophobic by adjuvant treatment. When there is a proper amount of solvent action and a proper concentration of nucleating agent, silver sulfide nuclei form in the antiabrasion layer, or at the top of the emulsion layer; metallic silver forms by silver transfer development from the unexposed silver halide and deposits on the nuclein to give a silver surface image. Various methods of silver transfer development that are found in US. application to Blake, Ser. No. 401,971, filed Oct. 6, 1964 (US. Pat. 3,490,905, Jan. 20, 1970), can be used to produce a silver surface image in the unexposed area.

The nucleating agent must be capable of diffusing into the antiabrasion layer, or the top of the emulsion layer, if there is no antiabrasion layer. Examples of this agent are lead, barium, sodium, potassium, cadmium, zinc and ferrous sulfides; sodium, potassium, lead and cadmium selenides, sodium and potassium thiocyanates; and thiourea; see US. Pat. 3,490,905, US. application to Strange, Ser. No. 494,940, filed Oct. 11, 1965 (now abandoned). The concentration of the nucleating agent in the developer solution varies with the type of light-sensitive element used in this process. The operable concentrations are readily determined in each case by relatively simple laboratory techniques, and optimum concentrations are determined in the same manner.

The silver halide solvent in the developer can be a thiosulfate or a thiocyanate compound and has the function of dissolving and transporting silver to the nuclei where it is reduced to form a silver image.

The most preferred embodiment of the process of this invention is as follows:

(A) A light-sensitive silver halide emulsion on a film support is exposed through the film support or to the emulsion side in a conventional camera with an optically reversing prism to give a negative exposure.

(B) The exposed film is developed in a silver transfer developer containing hydroquinone, 1-phenyl-3-pyrazolidone, sodium sulfide, potassium thiocyanate, and other normal developer ingredients.

(C) The developed film is washed in water.

(D) The emulsion side of the wet, developed film is treated with an ethanol solution containing an adjuvant having a hydrophilic group to prepare a lithographic printing plate.

(E) The lithographic printing plate is placed on an offset duplicating press, wetted with water, inked with an ink containing an oleophilic binding agent, and used to print negative copies.

4 The following examples illustrate this invention but not intended to limit it in any way.

EXAMPLE 1 A mono-bath developer solution, Solution A, was prepared as follows:

Solution A Water (distilled)600 gm.

Sodium sulfite (anhydrous)-33.3 gm.

Hydroquinone (anhydrous)l gm.

Ethylene glycol66.7 gm.

l-phenyl-3-pyrazolidone (reagent grade)l.0- gm.

N-methyl-Z-pyrrolidone (reagent grade)-3.3 ml.

Potassium carbonate22.6 gm.

Bcnzotriazole-0.5 gm.

Sequestrene (trisodium salt of ethylene diamine tetraacetic acid)-0.l6 gm.

Sodium sulfide (0.1 molar solution)10 ml.

Potassium thiocyanate (70 gm. dissolved in sufiicient water to make up 100 ml. of solution)-l5 ml.

Sodium hydroxide (3 molar solution)-108 ml. Water to make 1 liter.

A liter of another developer, Solution B, was prepared Solution B was identical in composition to Solution A except that sodium sulfide and potassium thiocyanate were omitted.

Three pieces of photographic film of the lithographic type were exposed at a distance of 24 inches for seconds through a high-contrast photographic positive transparency in contact with the film of a high-intensity, tungsten-filament, incandescent lamp (No. 2 General Electric Photofiood lamp) operated at 17 volts A.C. The film had (a) a high-contrast silver chloride-silver bromide orthochromatically sensitized emulsion, in which the silver salt contained 70 mole percent silver chloride and mole percent silver bromide, and contained grams of gelatin per mole of silver halide, (b) was overcoated with a clear, hardened gelatin layer, and (c) a base coated with a dyed gelatin-NH (nonhalation) backing.

One piece of exposed film was immersed in Solution A for one minute and washed with water for three minutes. A high-contrast, highly-reflective, positive silver image was obtained on and in the antiabrasion surface coating of the emulsion layer. The film was then treated on the emulsion side with a thiouracil solution (one gram of thiouracil dissolved in sufficient denatured ethanol to make up one liter of solution), and washed with water after the thiouracil treatment. The film constituted a printing plate. Using a conventional greasy (oleophilic) lithographic ink, a hand roller was inked which, in turn, was used to ink the Wet treated film. A sheet of paper was pressed against the inked film and a negative copy having good image quality and unspeckled background was obtained. Several more negative copies having good image quality and unspeckled background were obtained using the same technique.

The second piece of exposed film was immersed in Solution B for one minute and washed with water for three minutes. A normal negative silver image was obtained within the emulsion layer. This exposed film was used as a control. The same printing method was used and several negative copies having very poor image quality were obtained from the first control.

The third place of exposed film was similarly developed in Solution A and Washed. The film was treated with an ethanol solution (no adjuvant), washed with water and used as a second control. The same printing method was used and several negative copies having poor image quality were obtained from this second control.

EXAMPLE 2 Example 1 was repeated except that 6-rn-ethyl thiouracil was substituted for the adjuvant. Several negative copies having good image quality and unspeckled background were obtained from the treated film. When the control films were used as printing plates, negative copies having very poor quality were obtained.

EXAMPLE 3 The first solution, Solution C, of a dual-bath developer was prepared as follows:

Solution C Water600 ml. Sodium sulfite (anhydrous)-80 gm. Hydroquinone (anhydrous)16 gm. 1-phenyl-4-methyl-3-pyrazolidone (reagent grade) 1 gm. Boric acid (H BO (crystals)5.5 gm. Sodium hydroxide (in 200 ml. of water)24 gm. S-nitrobenzimidazole nitrate (2 gm. dissolved in 100 ml. 1r

of 1 to 1 ethanol-water solution) ml. Water to make 1 liter A photographic film of the lithographic type as described in Example 1 was exposed as described in Example 1. The film was immersed in Solution C for 30 seconds, washed with water for two minutes, immersed in Solution D for 30 seconds and washed with water for five minutes. The film was then treated on the emulsion side with a mercapto acetic acid solution (one gram of mercapto acetic acid dissolved in sufficient 95% denatured ethanol to make up 100 ml. of solution), and washed with water after the mercapto acetic acid treatment. The film constituted a printing plate. Using a conventional greasy (oleophilic) lithographic ink, a hand roller was inked which, in turn, was used to ink the wet treated film. A sheet of paper was pressed against the inked film and a. negative copy having good image quality and unspeckled background was obtained. Several more negative copies having good image quality and unspeckled background were obtained using the same technique.

The technique of using two baths as set forth in this example is preferred since it avoids the possibility of forming developing surface silver image in the exposed areas.

EXAMPLE 4 Example 3 was repeated except that thiomalic acid was used as the adjuvant in place of the mercapto acetic acid. Several negative copies having good image quality and unspeckled background were obtained from the adjuvant treated film.

EXAMPLE 5 Example 3 was repeated except that a dilferent developer solution was used for the first bath and that thiouracil was used as the adjuvant. Solution E was used as the first developer solution and was prepared as follows:

Solution E Water600 ml.

Sodium sulfite (anhydrous) gm.

l-methyl-amino 4 hydroxybenzene sulfate (reagent r grade)--12 gm. 0

2,4-diaminophenol (reagent grade)9.4 gm. Boric acid (H BO (crystals)5.5 gm. Sodium hydroxide (7.5 gm. dissolved in suflicient water to make up ml. of solution)100 m1. S-nitrobenzimidazole nitrate (2 gm. dissolved in 100 ml.

of a 1 to 1 ethanol-water solution)20 ml. Water to make 1 liter Several negative copies having good image quality and unspeckled background were obtained from the adjuvanttreated film.

EXAMPLE 6 Example 2 was repeated except that a modified Solution E was used as the mono-bath developer. Before the components of Solution E as described in Example 5 were diluted with water to one liter, sodium sulfide (10 ml. of 0.1 molar solution) and potassium thiocyanate (15 ml. of a solution prepared by dissolving 70 gm. of potassium thiocyanate in sufficient water to make up 100 ml. of solution) were added. This modified Solution E was diluted to one liter and used as the mono-bath developer. Many negative copies having good image quality and unspeckled background were obtained from the adjuvant-treated film.

EXAMPLE 7 Example 1 was repeated except that tantalum potassium fluoride Was used as the adjuvant. The adjuvant solution was prepared by adding one gram of TaKzFq to 100 m1. of cold water. The TaK2F7 did not dissolve entirely. The remaining solids in the solution were allowed to settle, and then the solution was decanted otf. After the adjuvant was applied to the developed film, several negative copies having good image quality and unspeckled background were obtained.

EXAMPLE 8 Example 7 was repeated except that a diiferent printing method was used. The TaK F -treated film was placed on a conventional oifset duplicating press, and inked with a conventional greasy lithographic ink. Using a 5% acetic acid solution as the fountain solution, many negative copies having good image quality and unspeckled background were obtained.

EXAMPLE 9 Example 3 was repeated except that tantalum potassium fluoride was used as the adjuvant. The adjuvant solution was prepared as described in Example 7. Several negative copies having good image quality and unspeckled background were obtained.

EXAMPLE 10 A photographic film of the type described in Example 1 was exposed and developed as described in Example 3 and no adjuvant bath was used. Using a conventional greasy lithographic ink, a hand roller was used to ink the wet developed film. A sheet of paper was pressed against the inked film and a negative copy having very poor image quality was obtained. Several more negative copies of similar poor quality were made. The film was then treated with tantalum potassium fluoride solution (made as described in Example 7), and several negative copies having good image quality and unspeckled background were obtained.

EXAMPLE 11 Example 1 was repeated except that Solution A contained 22.5 ml. of potassium thiocyanate (0.7 g./ml.), and the film after exposure and development as in Example 1 was treated with a saturated solution of 2-thiouracil-S-carboxylic acid in ethanol. Several negative copies 7 were produced as in Example l using the treated film. A sample of the film which was treated with ethanol only produced a poor positive copy instead of the desired negative copy.

Additional adjuvants of l-thioglycerol, thiosalicylic acid or Z-mercaptomethyl amine hydrochloride may be used in the above example.

Although the light-sensitive element is preferably transparent so that it can be exposed from either the front or back, an element with an opaque backing, support, etc., can be exposed from the front and subsequently developed so that there is a necessary silver transfer to the unexposed surface region.

The preferred method of exposure is in a camera through the base or to the emulsion using an image reversing prism so that there is lateral image reversal. Hence in that manner a correctly oriented positive copy can be obtained when the light-sensitive element is ultimately used as a printing plate.

The organic colloid of the silver halide emulsion layer used in this invention is not especially critical and may be gelatin, or, in place of gelatin, other natural or synthetic organic colloid binding agents. Such agents include waterpermeable or water-soluble polyvinyl alcohol and its derivatives, e.g., partially hydrolyzed polyvinyl acetates, polyvinyl ethers and acetals containing a large number of extralinear -CH CHOH- groups; hydrolyzed interpolymers of vinyl acetate and unsaturated addition polymerizable compounds such as maleic anhydride, acrylic and methacrylic acid ethyl esters and styrene. Sutiable colloids of the last mentioned type are disclosed in US. Pats. 2,276,322; 2,276,323 and 2,347,811. The useful polyvinyls include polyvinyl acetal aldehyde acetal, polyvinyl butyldiehyde acetal, and polyvinyl sodium o-sulfobenzaldehyde acetal. Other useful colloid binding agents include poly-n-vinyllactams of Bolton US. Pat. 2,495,918, the hydrophilic copolymers of N-acrylamide alkyl betaines described in Shacklett, US. Pat. 2,833,050, hydrophilic cellulose ethers and esters, colloidal albumin, zein and polyacrylamide.

The film support for the emulsion layers used in the novel process may be any suitable transparent plastic. For example, the cellulosic supports, e.g., cellulose acetate, cellulose triacetate, cellulose mixed esters, etc., may be used. Polymerized vinyl compounds, e.g., copolymerized vinyl acetate and vinyl chloride, polystyrene and polymerized acrylates may also be mentioned. The film formed from the polyesterification product of a dicarboxylic acid and a dihydric alcohol made according to the teaching of Allies, U.S. Pat. 2,779,684 and the patents referred to in the specification of that patent. Other suitable supports are the polyethylene terephthalate/isophthalates of British Pat. 766,290 and Canadian Pat. 562,672 and those obtainable by condensing terephthalic acid and dimethyl terephthalate with propylene glycol, diethylene glycol, tetramethylene glycol or cyclohexane 1,4-dimethanol (hexahydro-p-xylene alcohol). The films of Bauer et al., US. Pat. 3,052,543, may also be used. The above polyester films are particularly suitable because of their dimensional stability.

Paper is another example of a suitable support for this novel process of obtaining the planographic printing form, provided this paper is coated with a water-resisting layer, for example, with a hardened gelatin layer or superficially saponified cellulose acetate. Into such layers there may be incorporated matting agents such as titanium dioxide, silicon dioxide, barium sulfate, in varying quantities.

As the support for the planographic printing form, it is further possible to use plates consisting of metals, alloys or metal oxides coated metal.

The light-sensitive element may or may not contain an antiabrasion layer on top of the emulsion layer, although its presence is preferred.

Besides immersing the light-sensitive element in the developer or developers solutions, other methods of applying the solutions can be used, such as, dip roll, hopper, spray, and the like, as long as a sufiicie-nt amount of solution is applied to the surface.

Just prior to the adjuvant treatment, the printing plate can be treated with formaldehyde which hardens the emulsion and antiabrasion layers thereby lengthening the useful life of the printing plate.

There are several simple plate eradication methods that can be used with this printing plate that are effective in removal of unwanted portions of the printing image on the face of the plate. One very simple method of plate eradication is rubbing the area designated for removal with a cotton swab dipped in gum arabic or even an ordinary rubber eraser.

This novel process produces negative copies with excellent sharpness and gives rendition of the detail with a clean white background throughout prolonged use. The photolithographic process of the invention has the advantages that it is simple, effective, convenient and versatile.

A printing plate made by the process taught by this invention has the important characteristic of being able to immediately start producing useable copies. It also has the ability to immediately print satisfactory copies after a period of machine shutdown. The plate will permit the making of erasures without causing undesirable marks to appear. The plate is easy to make, inexpensive and easy to use for printing. All experiments showed faster start-up when the plate was used for printing than when untreated controls were used. The plate can be exposed and developed several months before it is treated with the appropriate adjuvant in preparation for its use as a printing plate.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for producing a lithographic printing plate from an image-exposed photographic element having a gelatino-silver halide layer bearing a normally developed silver image in the exposed areas and a complementary metallic silver surface image in the unexposed areas formed by silver transfer development, the improvement which comprises treating the surface of the element with an adjuvant solution which renders the unexposed areas more oleophobic than the exposed areas, said solution being (1) a solution of an organic adjuvant containing an oleophobic group and a group selected from the class consisting of:

(a) -SH or a group enolizable to SH,

(b) SR where R is an easily hydrolyzable group to give SH,

(c) a thioacid group,

(d) a thioamide group,

(e) a selenium analogue of groups (a) through (d), or (2) an aqueous solution of tantalum potassium fluoride.

2. A process as defined in claim 1 where said element has a Water permeable antiabrasion layer.

3. A process as defined in claim 1 where said adjuvant is thiouracil, mercaptosuccinic acid, thioacetic acid, 2- thiouracil-S-carboxylic acid, l-thiogly'c erol, o-mercaptobenzoic acid, 6-methyl-thiouracil, Z-mercaptomethyl amine hydrochloride or tantalum potassium fluoride.

4. A process as defined in claim 3 where the surface of said element after treatment with said adjuvant is wet with an aqueous fountain solution, an oleophilic ink is applied to the surface of said element and a copy of the orlginally exposed image is transferred to a support.

5. A process as defined in claim 3 where said adjuvant solution is an ethanol solution of thiouracil.

6. A process as defined in claim 3 where said adjuvant solution is an ethanol solution of mercaptosuccinic acid.

7. A process as defined in claim 3 where said adjuvant solution is an ethanol solution of 6-methyl thiouracil.

8. A process as defined in claim 3 where said adjuvant solution is an ethanol solution or thioacetic acid.

References Cited UNITED STATES PATENTS 1/1970 Blake 96--29 10 FOREIGN PATENTS 1,461,640 12/1966 France 9633 969,072 9/1964 Great Britain 9629 6 NORMAN G. TORCHIN, Primary Examiner J. WINKE-LMAN, Assistant Examiner US. Cl. X.R. 96-29, 66 

